Metals which are used to form apparatus comprising recirculating water systems, steam boilers, cooling towers and other related equipment are subject to the formation of scale which are water-formed deposits. The formation of scale and sludge deposits on the surfaces of these metals constitutes a serious problem involved in the operations of these various apparatus. In addition, the formation of scale, which may be largely attributed to the deposition of salts such as the carbonates, sulfates, phosphates, and hydroxides of calcium and magnesium on the heat transfer surfaces, adversely effects the heat transfer of the aqueous medium and concomitantly reduces the efficiency of the operation. The aforementioned salts of magnesium and calcium may be inherently present in untreated water which is utilized in the various operations.
Heretofore, various methods for removing scale deposits from the metal surfaces of, for example, a recirculating water system have been effected in many various ways. One method has consisted of treating the metal with a hydrochloric solution. However, during the acid cleaning procedure, extreme care must be observed in order to protect the metal against being eaten away or eroded by the acid. Another method has been to inhibit the formation of scale by the addition of a number of materials to the water. For example, polyphosphates have been added to cooling waters not only for preventing or inhibiting scale formation but also to prevent corrosion of the metal surfaces. The polymeric phosphates form water soluble complexes with the calcium and magnesium salts which are responsible for the hardness of the water by preventing the formation of firmly adhering scale covering by disturbing the growth of the crystal lattice. Other materials which have been used for the chemical treatment of water have included tannin, lignins, starches, polyacrylates and polymethacrylates, these compounds being used as sludge conditioners to render the sludge non-sticky and thus more readily manageable.
In addition to the aforementioned materials which are used for the inhibiting of corrosion or scale and in the treatment of equipment such as pumps, heat exchangers, cooling towers, etc. a scale inhibitor is disclosed in U.S. Pat. No. 3,962,110 which comprises a water-soluble polyvinylpyrrolidone having a molecular weight of from about 10,000 to about 1,000,000, said polyvinylpyrrolidone being used in an acidic system which has been made acidic by the addition of a non-oxidizing mineral acid such as hydrochloric acid, sulfuric acid, etc. U.S. Pat. No. 4,026,815 discloses a method for preventing corrosion in water-carrying systems utilizing as a corrosion inhibitor a phosphonocarboxylic acid and a benzimidazole derivative. In a like manner, U.S. Pat. No. 3,959,167 utilizes, as a composition for inhibiting the formation of scale or boiler water, a mixture of an acrylic polymer selected from the group consisting of polymethacrylic acid, polyacrylic acid, and water-soluble salts thereof, a water-soluble chelant such as nitrilotriacetic acid and hydroxyalkylidenediphosphonic acid. In addition, U.S. Pat. No. 4,255,259 also is drawn to a method for inhibiting scale in a boiler water system utilizing, as a scale inhibitor, a composition consisting of copolymers of styrene, sulfonic acid, and maleic anhydride or maleic acid with a water-soluble sulfonic acid.
As will hereinafter be shown in greater detail, it has now been discovered that the formation of scale in such equipment as water cooling towers will be greatly inhibited by utilizing the mixture of polyvinylpyrrolidone with aminotri(methylenephosphonic acid), the combination of these compounds resulting in a synergistic action as regards the formation of scale.